NSArray+FunctionalPrimitives.h

#import <Foundation/Foundation.h>

@interface NSArray (FunctionalPrimitives)

- (NSArray *)flatten;
- (NSArray *)reverse;
- (NSArray *)map:(id (^)(id))transform;
- (NSArray *)filter:(BOOL (^)(id))predicateBlock;
- (NSArray *)foldRightWithStart:(id)start reduce:(id (^)(id item, id result))reduce;
- (NSArray *)foldRightWithStart:(id)start reduce:(id (^)(id item, id result))reduce;
- (NSArray *)flatMap:(id (^)(id))transform;

@end

NSArray+FunctionalPrimitives.m

#import "NSArray+FunctionalPrimitives.h"

@implementation NSArray (FunctionalPrimitives)

- (NSArray *)flatten {
    NSMutableArray *result = [NSMutableArray new];
    
    for (id item in self) {
        if ([item isKindOfClass:[NSArray class]]) {
            [result addObjectsFromArray:[item flatten]];
        } else {
            [result addObject:item];
        }
    }
    
    return result;
}

- (NSArray *)reverse {
    NSMutableArray *results = [[NSMutableArray alloc] initWithCapacity:self.count];
    
    for (id item in [self reverseObjectEnumerator]) {
        [results addObject:item];
    }
    
    return results;
}

- (NSArray *)map:(id (^)(id))transform {
    NSMutableArray *results = [[NSMutableArray alloc] initWithCapacity:self.count];
    
    for (id item in self) {
        [results addObject:transform(item)];
    }
    
    return [results copy];
}

- (NSArray *)filter:(BOOL (^)(id))predicateBlock {
    return [self filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(id  _Nonnull evaluatedObject, NSDictionary<NSString *,id> * _Nullable bindings) {
        return predicateBlock(evaluatedObject);
    }]];
}

- (NSArray *)foldLeftWithStart:(id)start reduce:(id (^)(id, id))reduce {
    id result = start;
    for (id item in self) {
        result = reduce(item, result);
    }
    return result;
}

- (NSArray *)foldRightWithStart:(id)start reduce:(id (^)(id, id))reduce
{
    return  [[self reverse] foldLeftWithStart:start reduce:reduce];
}

- (NSArray *)flatMap:(id (^)(id))transform {
    return [[self map:transform] flatten];
}

@end